Up to now, nematic LC devices being used in reflective light valves for projection displays require either a linearly-polarized or randomly-polarized incident light beam. The invention concerns only with the case where the incident light is linearly polarized. The commonly used LC modes which require incident light being linearly polarized for reflective light valves are electro-control birefringence (ECB) mode with tilted homogeneous alignment, the deformation of aligned phase (DAP), the hybrid-field-effect (HFE) mode, the 63.6.degree.-twist mode.sup.3, hybrid-aligned namtic (HAN) mode, and the mixed TN (MTN) mode. The operating principles of these LC modes for reflective light valves are illustrated in FIG. 1. An incident light beam 6 after passing through a polarizing beam splitter (PBS) 7 becomes a linearly-polarized light 8, defined as p-wave, and impinges on a nematic LC cell 100 which contains a front substrate 1, a rear substrate 2, a nematic LC medium 5 in between, and two LC alignment directions 3 and 4 on the substrates 1 and 2, respectively. There are two electrodes (not shown in FIG. 1), one being a transparent electrode behind the front substrate 1 facing the LC medium 5, and the other being a reflective metal electrode (not shown in FIG. 1) in front of the rear substrate 2 facing the LC medium. The nematic LC cell 100 shown in FIG. 1 (b) is designed in such a way that at or below a certain voltage defined as the threshold voltage applied to the two electrodes of the the LC cell 100, the incident polarized beam 8 will become the s-wave 9 (or nearly s-wave) upon reflection from the LC cell 100. The s-wave is a linearly polarized light whose direction of polarization being perpendicular to that of the p-wave. The s-wave 9 will be reflected 90.degree. by the PBS 7 into the s-wave 10 to be collected by projection lenses (not shown in FIG. 1) onto a screen for viewing. This situation represents the bright state of the light valve. When an external voltage is applied across the two electrodes of the LC cell 100 such that at or above a certain voltage defined as the saturation voltage, the LC cell 100 will behave nearly as an optically isotropic medium. In this situation, the impinging linearly polarized light 8 will be reflected from the reflective LC cell 100 preserving the same direction of polarization, p-wave in this case. The reflected p-wave after passing through the PBS 7, will propagate backward opposite to the incident beam 6. The reflected p-wave has a negligible intensity being reflected 90.degree. by the PBS 7 into the s-wave 10, representing the dark state of the light valve. When the applied voltage level is in between the threshold voltage and the saturation voltage, inter-mediate gray level will be activated to achieve display with many gray levels up to 1024 levels.
The prior art of nematic LC modes for reflective light valves which require the incident light being linearly polarized are shown schematically in FIG. 2, which consists mainly of a front substrate 21, a rear substrate 22, a nematic LC medium 25, an LC director 23 adjacent to the front substrate 21, and a LC director 24 adjacent to the rear substrate 22. A x,y,z coordinate system is also shown in FIG. 2 to correlate with the x,y,z coordinate system shown in FIG. 1. There are two parameters, .alpha..sub.1 and .phi..sub.1 to describe the orientation of the LC director 23. The .alpha..sub.1 is the tilted angle of the LC director 23 from the plane of the front substrate 21, and .phi..sub.1 is the azimuthal angle of the LC director 23 projected onto the front substrate 21 with respect to the x-axis. There are also two parameters, .alpha..sub.2 and .phi..sub.2 to describe the orientation of the LC director 24. The .alpha..sub.2 is the tilted angle of the LC director 24 from the plane of the rear substrate 22, and .phi..sub.2 is the azimuthal angle of the LC director 24 projected onto the rear substrate 22 with respect to the x-axis. By choosing a nematic mixture with either a positive or a negative dielectric anisotropy and a set of parameters for .alpha..sub.1, .phi..sub.1, .alpha..sub.2, and .phi..sub.2, we can describe or define the six combinations of the prior art.
When properly optimized for nearly monochromatic incident light, in principle, the ECB mode, the HAN mode, and and the 63.6.degree.-twist mode have the same high optical efficiency of nearly 100% if anti-reflection films are coated on the window of the light valves. Here, we define optical efficiency as the conversion of the incident p-wave into reflected s-wave neglecting the loss due to indium-tin-oxide and the metal electrodes and the LC alignment layers in the light valves. However, all of the combinations except the ECB mode with tilted homogeneous alignment require rather high operating voltage, usually larger than 6 V. While the ECB mode with tilted homogeneous alignment can be operated at a lower voltage, it requires a stringent cell-gap uniformity. The MTN mode has a saturation voltage of about 4 V and a lower optical efficiency of 0.88 compared to other combinations for reflective light valves.
There are three important criteria for selecting the most suitable LC mode for reflective light valves. The first criterion is high optical efficiency. The second criterion is to have a large tolerance in cell-gap non-uniformity so that high yield can be achieved in manufacture. The third criterion is to have a low saturation voltage which is particularly important using active matrices on Si-wafer to drive the reflective light valves. The lower the saturation voltage is, the higher the display resolution can be achieved for a fixed area of Si-wafer. Using a LC mode with lower saturation voltage will result in lower cost in manufacture and low-power consumption. Law saturation voltage can be achieved using the ECB mode with tilted homogeneous alignment but its requirement on the cell-gap uniformity is too stringent. The MTN mode has a lower but not the lowest saturation voltage, and a large tolerance in cell-gap non-uniformity, but it has a poor optical efficiency of 0.88.
It is an object of the present invention to provide a self-compensated twisted nematic (SCTN) mode that has the advantages of low operating voltage, high optical efficiency, and a relatively large tolerance for cell-gap non-uniformity especially suitable for active-matrix-driven reflective light valves based on Si-wafers.